Industrial products such to continuing motion and friction need to be durable. Belts transfer power from one location to another. There needs to be sufficient friction to provide an efficient and productive transfer of power but not so much friction as to wear out the belt prematurely.
Belts are often made of vulcanized rubber or polyurethanes. Both types of belts are costly to produce because the vulcanization or polymerization processes are complicated and irreversible.